Veterinarians have a vast and ever-expanding array of diagnostic tests available to them. However, this abundance can be an embarrassment of riches that confounds diagnosis and undermines patient care if we do not make critical and informed decisions about the selection and interpretation of the tests we employ. Effective use of diagnostic tests requires a deliberate and informed approach. We must consider the strengths and weaknesses of the tests themselves and the clinical context, and we must be wary of the many biases that skew our use and interpretation of diagnostic tests. Understanding sensitivity and specificity, likelihood, prevalence and predictive value, the basic principles of Bayesian reasoning, and the cognitive biases that drive inappropriate testing are all critical to ensuring our use of imaging and laboratory testing improves patient outcomes.